Late onset Alzheimer's disease (AD) is the most common form of dementia, and is characterized by initial memory loss and then a progressive decline in cognitive function. Members of the Accelerating Medicines Partnership-Alzheimer's Disease (AMP-AD) program have exhaustively profiled gene expression in multiple brain regions from multiple cohorts of AD and control subjects, and have then performed systems biology analyses to identify molecular networks and drivers implicated in late onset AD. VGF (non-acronymic) is one of the top ranked AD drivers identified by several groups. Moreover, biomarker studies have consistently identified reduced VGF levels in the brains and CSF of patients with neurodegenerative disease including AD, and show that VGF is also a strong candidate biomarker of AD progression, with an estimated 10% decrease in CSF levels of VGF per year in diseased patients but not in age-matched controls. We show that VGF overexpression in hippocampus or chronic intracerebroventricular (icv) infusion of the VGF-derived neuropeptide TLQP-21 (named by its N-terminal 4 amino acids and length) reduces cortical and hippocampal amyloid deposition, microgliosis, and astrogliosis, and cognitive impairment in the 5xFAD mouse model of amyloidosis (Beckmann et al., under review). TLQP-21 activates the complement C3aR1 G-protein coupled receptor (GPCR), a regulator of AD pathogenesis that is expressed in the CNS on neurons, microglia, and astrocytes. The mechanism(s) of action of TLQP-21 with respect to AD neuropathology will be further investigated in our proposed studies, and differences between C3a and TLQP-21 in the regulation of microglial function via C3aR1 will be identified in vitro and in vivo. Three specific aims are proposed: (1) To determine the mechanism(s) by which VGF-derived peptide TLQP-21 activates C3aR1 and modulates function in microglia, and to investigate how these signaling outcomes differ compared to C3a/C3aR1; (2) To determine the underlying pathways by which VGF and VGF-derived C-terminal peptides TLQP-21 and TLQP-62, a C- terminally extended peptide overlapping TLQP-21 that does not activate C3aR1, block or delay development of neuropathology and cognitive impairment in mouse PS19 tauopathy and 5xFAD amyloidopathy models; (3) To determine whether C3aR1 expression on microglia, astroglia, and/or neurons is required for VGF/TLQP-21 efficacy to reduce amyloid load, microgliosis, and/or astrogliosis in the 5xFAD mouse model. In addition to measuring the progression of amyloid and tau pathology, we will employ microglial-specific and bulk RNA seq approaches throughout our studies to construct signaling networks that will help us determine how altered VGF or TLQP-21 levels impact microglial function and gene expression, in 5xFAD and PS19 mouse models with germline or conditional ablation of C3aR1. Comparison of these networks to integrative genomic efforts (AMP- AD) that map networks underlying the onset and progression of human AD may identify novel targets and pathways for pharmacotherapeutic intervention in AD.